Hitherto, many resin-sealing type semiconductor devices have been resin-sealed by a transfer molding method. The transfer molding method comprises injecting a molten resin in a die cavity, in which a semiconductor element and a part of the lead frame are placed, by means of a transfer molding machine, to thereby resin-seal the semiconductor element and a part of the lead frame. The transfer molding method has been widely used for not less than ten years, the technique of the method has been established, and the transfer molding method can be said to be a stably mass-producible method about the semiconductor devices obtained by resin-sealing conventional general semiconductor elements.
However, recently, with the increase of the so-called environmental problem, the low utilization efficiency of the sealing resin in the transfer molding method is becoming a problem. In the transfer molding method, the utilization efficiency of the sealing resin is only from about 50 to 70%, the disadvantage of generating a large amount of a resin waste is pointed out, and thus the immediate improvement has been desired.
As a technique for solving such a problem, a method of forming a label-form or sheet-form sealing resin having a uniform thickness and sticking the sealing resin to a semiconductor element by heat-pressing is proposed. Also, it is proposed to improve the moisture-proof reliability and the strength of semiconductor devices by using a sealing label prepared by forming the sealing resin on a metal foil or a metal plate uniformly as described in JP-A-4-340258 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
By the above-described resin sealing method for semiconductor elements using a sealing resin formed into a label form or a sheet form, the utilization efficiency of the sealing resin becomes 100% in principle and the resin waste is not generated in the production process. However, in the case of using the method, there is a disadvantage that a void is liable to form in the resin after sealing and the point becomes the cause for delaying the practical use of the sealing method.
In addition, for solving the above-described problem, a die side is devised in such a manner that a die used at sticking the label-form or sheet-form sealing resin to a semiconductor element is divided into a frame-form die surrounding the circumstance of the sealing resin label or sheet and a press die setting in the inside of the frame-form die and an air-bent groove extending to the direction of crossing at right angle with the lead frame is formed on the inside surface of the frame-form die or the outside surface of the press die as described in JP-A-5-343458. However, in the method, not only it is necessary to use the complicated dies but also it cannot be said that the effect of preventing the formation of void is sufficient.